Physics News Update
The American Institute of Physics Bulletin of Physics News

Number 337 September 18, 1997 by Phillip F. Schewe and Ben Stein

THE SMALLEST NONZERO BRANCHING RATIO EVER
MEASURED. Most known particles are highly fragile. Not only
do they decay quickly but through a myriad of ways. Calculating
and then also measuring the relative likelihoods (branching ratios)
of the different forms of particle mortality are important
diagnostics for understanding how matter behaves at the most
fundamental level. A particularly rare form of decay (a very tiny
tributary to the Amazon of expiring particles, as it were) is the
decay of the K+ meson into a pi+ meson, a neutrino, and an
anti-neutrino, a process sometimes involving the momentary
creation of both a charged W boson (the carrier of the weak
nuclear force) and its neutral cousin, the Z boson (which itself
instantly decays into the two neutrinos), rather than the more
common exchange of a single W or Z. A large collaboration at
Brookhaven (contact Douglas Bryman, doug@triumf.ca, 604-
222-7338) has examined more than a trillion K decays and, after
years of painstaking vigilance, they have finally found one event
with the telltale signature. The expected background for this
process, for their amount of data, would be 0.08 events. The
researchers are therefore confident that they have measured a true
K decay branching ratio, with a value of 4.2 x 10-10 (with an
uncertainty of +9.7 or -3.5 x 10-10). In the next year or so
additional data should settle the issue of whether the observed
rare K decay conforms to the standard model of particle physics
(in which case the measured decay rate will provide information
about related processes, such as the decay of top quarks) or
represents evidence of "new physics" outside the current theory.
(S. Adler et al.,
Physical Review Letters,
22 September 1997;
see also a
figure at
Physics News Graphics)

REAL PHOTONS CREATE MATTER. Einstein's equation
E=mc2 formulates the idea that matter can be converted into
light and vice versa. The vice-versa part, though, hasn't been so
easy to bring about in the lab. But now physicists at SLAC have
produced electron-positron pairs from the scattering of two "real"
photons (as opposed to the "virtual" photons that mediate the
electromagnetic scattering of charged particles). To begin, light
from a terawatt laser is sent into SLAC's highly focused beam of
47-GeV electrons. Some of the laser photons are scattered
backwards, and in so doing convert into high-energy gamma ray
photons. Some of these, in turn, scatter from other laser
photons, affording the first ever creation of matter from light-on-
light scattering of real photons in a lab. (D.L. Burke et al.,
Physical Review Letters, 1 September 1997.)

DNA-GOLD NANOPARTICLES, employing the talent of DNA
strands for recognizing and attaching to complementary strands
and gold's electronic and optical properties, operate as a new
kind of biosensor. Scientists at Northwestern University glue
various "probe" DNA segments onto tiny gold particles (13 nm
wide). When a "target" single-stranded DNA introduced into the
solution happens to be complementary to DNA already stuck to
the particles, the probe and target strands link up, creating a sort
of polymer network whose color is different from that of the
original solution. Thus recognition of the target DNA is signaled
by a color change. The researchers can already use this approach
to detect single-strand DNA samples in 10-femtomolar amounts.
(Science, 22 August 1997.)